Conventionally, when the collector voltage of pnp or npn transistors used as switching elements installed outside of the IC in switching regulators, etc., is changed (increased or decreased) at high speed, a high speed operation has been done by temporarily increasing the base current of the external transistor by adding an external capacitive element (capacitor) to the drive circuit.
FIG. 5 is a circuit diagram which illustrates the first structural example of a conventional overdrive circuit.
In FIG. 5, I.sub.e1 is the current source, Q.sub.1, Q.sub.2 are npn transistors, D.sub.1 is a diode, R.sub.1 is a resistance element, C.sub.1 is an external capacitor, QPT.sub.1 is an external pnp transistor, SD.sub.1 is a Schottky diode, L.sub.1 is a coil, C.sub.2 is a capacitor, V.sub.CC is the power source voltage, T.sub.1, T.sub.2, and T.sub.3 are the input/output terminals of the IC (IC terminals, hereafter).
In this circuit, the current source I.sub.e1, npn transistors Q.sub.1, Q.sub.2, diode D.sub.1, and the resistance element R.sub.1 are formed inside of the IC, and each element is connected as follows:
That is, the collector and the base of the transistor Q.sub.1 are connected to the current source I.sub.e1, and the emitter is connected to the anode of the diode D.sub.1. The cathode of the diode D.sub.1 is grounded.
The connection midpoints of the collector and the base of the transistor Q.sub.1 are connected to the base of the transistor Q.sub.2. The collector of the transistor Q.sub.2 is connected to the IC terminal T.sub.1, the emitter is connected to one end of the resistance element R.sub.1 and the IC terminal T.sub.2, and the other end of the resistance element R.sub.1 is grounded.
The electrode at one side of the external capacitor C.sub.1 is connected to the IC terminal T.sub.2, and the other electrode is connected to the IC terminal T.sub.3.
The emitter of the external transistor QPT.sub.1 is connected to the supply line of the power source voltage V.sub.CC, the base is connected to the IC terminal T.sub.1, and the collector is connected to the cathode of the Schottky diode SD.sub.1 and one end of the coil L.sub.1. The anode of the Schottky diode SD.sub.1 is grounded, the other end of the coil L.sub.1 is connected to one electrode of the capacitor C.sub.2, the other electrode of the capacitor C.sub.2 is grounded, and the connection midpoint of the other end of the coil L.sub.1 and one electrode of the capacitor C.sub.2 is connected to a load not illustrated in the figure.
In such a structure, the electric current from the current source I.sub.e1 is supplied to the collector and the base of the transistor Q.sub.1, and the base of the transistor Q.sub.2.
In this manner, both transistors Q.sub.1 and Q.sub.2 will be on, and the base emitter voltage V.sub.BE portion of the diode D.sub.1 will be impressed on both ends of the resistance terminal R.sub.1 as the voltage V.sub.1.
At this time, in the initial state, while the charge flows into the transistor Q.sub.2, the overdrive current I.sub.OVR such as illustrated in FIG. 6 will flow into the external capacitor C.sub.1, and this current is supplied to the base of the external transistor QPT.sub.1.
Therefore, the collector voltage V.sub.P1 of the external transistor QPT.sub.1 will rapidly rise as illustrated in FIG. 7.
In this manner, high-speed operation is realized and conversion efficiency will increase.
FIG. 8 is a circuit diagram illustrating the second structural example of a conventional overdrive circuit.
In FIG. 8, I.sub.e2 is a current source, P.sub.1 is a pnp transistor, Q.sub.3 and Q.sub.4 are npn transistors, D.sub.2 and D.sub.3 are diodes, R.sub.2 is a resistance element, C.sub.3 is an external capacitor, QPT.sub.1 is an external pnp transistor, SD.sub.1 is a Schottky diode, L.sub.1 is a coil, C.sub.2 is a capacitor, V.sub.CC is power source voltage, and T.sub.1, T.sub.2, and T.sub.3 indicate input/output terminals of the IC.
In the structure of this circuit, the transistors Q.sub.1 and Q.sub.2 and the diode D.sub.1 in the circuit in FIG. 5 are replaced by the diode D.sub.3, transistor P.sub.1, and diode D.sub.2. The external capacitor C.sub.3 and the resistance element R.sub.2 play similar roles to those of the external capacitor C.sub.1 and the resistance element R.sub.1 in FIG. 5. The connecting relationship between each element in the IC is different from that in the circuit in FIG. 5.
That is, the anode of the diode D.sub.2 is connected to the power source voltage V.sub.CC, and the cathode is connected to the anode of the diode D.sub.3. The cathode of the diode D.sub.3 is connected to both the current source I.sub.e2 and the base of the transistor P.sub.1.
The emitter of the transistor P.sub.1 is connected to one end each of the resistance element R.sub.2 and the IC terminal T.sub.3, and the collector is connected to both the collector and the base of the transistor Q.sub.3. The other end of the resistance element R.sub.2 is connected to the power source voltage V.sub.CC and the IC terminal T.sub.2.
One electrode of the external capacitor C.sub.3 is connected to the IC terminal T.sub.2, and the other electrode is connected to the IC terminal T.sub.3.
The emitter of the transistor Q.sub.3 is grounded, and the connection midpoint between the collector and the base is connected to the base of the transistor Q.sub.4. The collector of the transistor Q.sub.4 is connected to the IC terminal T.sub.1, and the emitter is grounded.
In the circuit in FIG. 8, when the electric current from the current source I.sub.e2 begins to flow, in the initial state, during the time while the charge of the external capacitor C.sub.3 flows out via the transistor P.sub.1, the overdrive current I.sub.OVR as illustrated in FIG. 9 will flow into the collector of the transistor P.sub.1.
That is, with regard to the collector current I.sub.P1 of the transistor P.sub.1, as illustrated in FIG. 9, the overdrive current I.sub.OVR will flow temporarily. Such a collector current I.sub.P1 of the transistor P.sub.1 is amplified by the transistors Q.sub.3 and Q.sub.4, which constitute a current mirror circuit, and is supplied to the base of the external transistor QPT.sub.1 as the current I.sub.Q4.
Therefore, the collector voltage V.sub.P1 of the external transistor QPT.sub.1 will rise quickly as illustrated in FIG. 7, and consequently, high speed operation is realized, and the conversion efficiency will increase.
Recently, in the field of portable equipment such as video cameras, the trend is to make the mounting area smaller by reducing as many external parts of the IC as possible.
However, with regard to the aforementioned conventional circuits, several hundred to several thousand pF will be needed as the capacitance for the capacitor C.sub.1 in the circuit in FIG. 5, and several tens to several hundred pF will be needed as the capacitance for the capacitor C.sub.3 in the circuit in FIG. 8. While it is possible to form a capacitor of several tens of pF inside the IC, this will result in an increased chip area, and consequently an increase in the IC cost. Therefore, it is inevitable that the aforementioned capacitors are attached outside the IC, meaning that a structure which is not desirable for the actual situation will be adopted, and which is a reason why the equipment is made larger.
It is an object of the present invention to provide an overdrive circuit that can have the number of external parts decreased without increasing the chip area or the IC cost.